This invention relates generally to improvements in free piston power units of the type having a pair of power pistons linked for reciprocatory movement in opposite directions at the opposite ends of a common power cylinder. More particularly, this invention relates to an improved free piston power unit having specially contoured bounce cylinders designed for providing pneumatic reaction forces to return the power pistons through a compression stroke while protecting against undesirably high pressures or resultant high temperatures, and further while permitting the entire power unit to fit within a compact overall envelope size.
Free piston power units in general are well known in the art to include a pair of power pistons mounted for reciprocatory movement within the opposite ends of a common, open-ended power or combustion cylinder. The two power pistons are mechanically linked in a suitable manner for reciprocation together in opposite directions within the power cylinder to expand or contract the volume of the power cylinder. A typical free piston machine is adapted for two stroke operation wherein the power pistons are displaced through an inward or compression stroke to compress a combustible mixture of fuel and air within the power cylinder, followed by combustion and power piston displacement through an outward or expansion stroke to perform useful work. In one common application of a free piston power unit, the power pistons are coupled to a respective pair of compressor pistons which are reciprocated by the power pistons within appropriate compressor cylinders to generate a supply of compressed gas, such as compressed air or the like.
In many prior art free piston power units, the addition of so-called bounce cylinder arrangements has been proposed to provide reaction forces which act at the end of an expansion or power stroke to return the power pistons through an inward compression stroke preparatory to a subsequent combustion cycle. Such bounce cylinder arrangements normally include a pair of bounce cylinders and associated bounce pistons located at coaxial outboard positions relative to the power pistons. The pistons are respectively coupled to the bounce cylinders or bounce pistons in a manner reducing bounce cylinder volume each time the power pistons are driven through an outward power stroke. A controlled quantity of gas, such as air, within each bounce cylinder is thus compressed during the outward power stroke, wherein this compressed gas within each bounce cylinder expands at the termination of the power stroke to return the power pistons through the inward compression stroke.
While bounce cylinder arrangements have beneficially enhanced the overall operating characteristics of free piston power units, such bounce cylinder arrangements undesirably increase the overall length of the machine. In the past, undesirable hign pressures or temperaures within the bounce cylinders have been addressed by utilizing bounce cylinders of extended length exposed to external cooling or by incorporation of special control systems for limiting bounce cylinder pressure. As previously stated, extending the bounce cylinder length undesirably increase overall machine size, whereas the use of special pressure control systems has been generally complex, costly, and for the most part ineffective.
There exists, therefore, a significant need for an improved free piston power unit having reaction force bounce cylinders, wherein the bounce cylinders are designed with relatively increased minimum cylinder volume to limit bounce cylinder pressures and temperatures, without increasing the overall size of the machine. The present invention fulfills these needs and provides further related advantages.